1. Field of the Invention
The present invention relates to a tracking servo apparatus for use with information-recorded disk players.
2. Description of the Background Information
The tracking servo apparatus is an indispensable part of disk players playing recording mediums such disks as video and compact audio disks (hereinafter simply referred to as disk). The servo apparatus controls the position of an information reading spot of the disk player's pickup so that a recording track of the disk currently being played is always followed accurately irrespectively of any disk defects or eccentricities that may exist.
In operation, the tracking servo apparatus generates a tracking error signal in response to the degree of defect or eccentricity of the information reading spot in the disk radius direction with respect to the recording track on the disk. In accordance with the tracking error signal, an actuator is driven to shift the information reading spot in the disk radius direction. This is a closed loop control system that provides positional control over the recording tracks on the disk.
Where a jump is to be made to a particular track on the disk, the tracking servo apparatus opens the servo loop and applies kick pulses to the actuator. At a predetermined point in time during the jump, e.g., at the zero crossing of the tracking error signal which occurs when the reading spot is positioned in the middle between tracks, brake pulses are applied to the actuator. Then at a suitable point in time, the servo loop is closed and the servo mechanism is retracted.
The track pitch on the disk is fixed according to predetermined standards. Thus for a one-track jump under track jump control, which was described above, the pulse width of kick pulses is set as desired within the period up to the zero crossing of the tracking error signal, whereas the pulse width of brake pulses is predetermined as constant relative to the track pitch. That is, the braking energy determined by the pulse width and peak value of the brake pulses remains constant.
However, an eccentric disk has varying jump distances according to the degree of eccentricity despite the fixed track pitch. For this reason, with the pulse width and peak value of brake pulses held constant, the braking energy gets too high or too low, making it impossible to perform stable jump operations. In FIG. 3, part (a) shows a case where no disk eccentricity exists. In this case, stable jump operations are carried out by the kick pulses having the pulse width and peak value depicted as well as by brake pulses in response to a tracking error signal TE.
The part (b) of FIG. 3 shows a case where the disk has forward eccentricity in the jump direction, the braking energy is too high despite the fact that the information reading spot moves at a low speed relative to the disk in the radius direction on the disk. The part (c) of FIG. 3 shows a case where the disk has reverse eccentricity in the jump direction, the braking energy is too low despite a high relative speed of the information reading spot in the disk radius direction.